1. Field of the Invention
The present invention relates to a method of driving an antiferroelectric liquid crystal device and, more specifically, to a method of driving an antiferroelectric liquid crystal device by using thin film transistors to implement such driving.
2. Description of the Related Art
Recently there has been discovered an antiferroelectric liquid crystal phase that shows switching among three stable states (A. D. L. Chandani, et al., Jpn. J. Appl. Phys., 27, L729 (1988)), triggering the discussion on new display systems using this liquid crystal phase. Among several types of antiferroelectric liquid crystals that have been reported, the antiferroelectric liquid crystal phase that corresponds to the smectic C phase may be considered the most practical, and were reported to have been made by most of the latest researches. Its notation differing among researchers, this antiferroelectric liquid crystal is represented as, for example, Sy* phase (Japanese Patent Laid-Open Publication HEI 1-213390) or SmC.sub.A * (Fukuda, Literature of the 45th Joint Research by the 142nd Committee of the Japan Society for the Promotion of Science, p. 34 (1989)). It will hereinafter be represented as SmC.sub.A *. Although it is reported that this SmC.sub.A * phase has a spiral structure in bulk state (Fukuda, Literature of the 45th Joint Research by the 142nd Committee of the Japan Society for the Promotion of Science, p. 34 (1989)), it is also said that the phase will show such a molecular arrangement as shown in FIG. 1 (a) if the spiral is undone, for example by sealing the SmC.sub.A * phase into a liquid crystal cell thinner than the pitch length of the spiral. In more detail, the molecular arrangement is such that dipoles are oppositely directed layer by layer to cancel each other, causing molecules to be tilted in each reversed direction layer by layer. If an electric field is applied to this state, the molecular arrangement results in one in which the dipoles are aligned with the direction of the electric field, as shown in FIG. 1 (b) or (c). The relationship between the applied voltage and the tilt angle is as shown in FIG. 2. Liquid crystals can be in any of three stable states 1 to 3 and will draw the hysteresis curve depending on the relation between tilt angle and applied voltage, thus allowing the display function to be implemented using this relation. Accordingly, for example, display in contrast between light and shade can be done by combining polarizing plates with the display surface of a liquid crystal display device. For instance, by aligning the polarizing axes of a pair of polarizing plates put into the Cross-Nicol state with the layer normal line of the smectic layer of the antiferroelectric liquid crystal phase, such a voltage--transmittance curve can be obtained as shown in FIG. 3 (a).
Lately, compounds that show the SmC.sub.A * phase have been reported including the following compounds (M. Johno et al., Proc. Japan Display '89. p. 22 (1989)): ##STR1##
These compounds will not show the SmC.sub.A * phase at room temperature; however, by providing a liquid crystal composition in which the above compounds are mixed, it is made possible to obtain a material that shows the SmC.sub.A * phase in a wider temperature range around room temperature.
A matrix type liquid crystal device incorporating antiferroelectric liquid crystals has also been reported (M. Yamawaki et al., Japan Display '89, p. 26 (1989); Japanese Patent Laid-Open Publication HEI 3-125119; etc.). As one method to provide an antiferroelectric liquid crystal device, electrodes, orientation films, and the like are formed on a pair of substrates, and antiferroelectric liquid crystal material is sandwiched by the substrates, thus constituting an antiferroelectric liquid crystal device. Such an antiferroelectric liquid crystal device has advantages, such as a wide angle of visibility and high-speed response, similar to ferroelectric liquid crystal devices. Also, the antiferroelectric liquid crystal device has further advantages of being free from burning, high resistance to shocks, and the like, as compared to the ferroelectric liquid crystal device. However, when antiferroelectric liquid crystals are used to provide a matrix type liquid crystal display device, sufficient display cannot be expected unless some driving method appropriate to the properties of the antiferroelectric liquid crystals is incorporated. Some reports have been made upon the driving of antiferroelectric liquid crystals (M. Yamawaki et al., Japan Display '89, p. 26 (1989); Japanese Patent Laid-Open Publication HEI 3-125119; etc.). However, these driving methods are incapable of attaining sufficiently high contrast, incapable of providing multi-tone display, and have difficulty in such large-capacity display as to have more than 1000 scanning electrodes, disadvantageously.
The reason why the methods cannot attain sufficiently high contrast is that it would be actually quite difficult for the antiferroelectric liquid crystals to attain such an ideal voltage--transmittance curve as shown in FIG. 3 (a); practically, the antiferroelectric liquid crystals allow light to pass therethrough even at a low electric field strength as shown in FIG. 3 (b), making it difficult to attain a sufficient black display.
The reason why the methods cannot provide multi-tone display is that the simple matrix type driving in which antiferroelectric liquid crystals are applied utilizes switching among the three stable states and therefore cannot make use of the intermediate states therebetween.
The reason why the methods have difficulty in fabricating such large-capacity display devices as to have more than 1000 scanning electrodes is as follows: Implementing such driving as will be free from flickers requires the frame cycle to be not less than 60 Hz. For example, in the case of 60 Hz, one frame is allotted 16.7 msec; if the number of scanning electrodes is 1000, the write time per scanning electrode results in 16.7 .mu.sec (=16.7 msec.div.1000). Although the antiferroelectric liquid crystals is required to have a response speed higher than that, the actual response speed of the antiferroelectric liquid crystal phase is slower (M. Johno et al., Proc. Japan Display '89, p. 22 (1989)), so that it is difficult to fabricate such large-capacity display devices as to have more than 1000 scanning electrodes.